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Geraniums are frost tender, so the last spring frost is the earliest possible date, but it is not the right one. The plant sits and does nothing while nights are cold, and a geranium planted out into 45F (7C) nights turns reddish, stops growing and stays that way for a month. Wait until nights stay above 50F (10C) and preferably 55F (13C).
A plant that waits an extra fortnight in its pot in a sheltered spot will overtake one put out early, and it will start flowering sooner. There is nothing to gain from rushing this plant into cold ground.
Harden off over about a week before planting, even for a plant that has only been on a windowsill. Start with a few sheltered hours outdoors, build to a full day, and give it two nights outside before it goes in the ground. Leaves moved straight from indoors to full sun scorch in pale papery patches.
Choose a pot that looks slightly too small
The instinct to give a healthy plant a generous pot works against you here. A geranium fills a large volume of fresh compost with root and foliage before it turns to flowering, and a plant potted three sizes up can spend a whole summer doing exactly that. Move up one size only.
A single well grown plant flowers well in a five to six inch (13 to 15 cm) pot and can stay there for two seasons. For a container display, plant three in a twelve inch (30 cm) pot rather than one, and expect far more flower for the space. Window boxes suit geraniums particularly well because the root volume is naturally restricted.
Drainage matters more than the pot material. Whatever you use must have open holes and must never stand in a saucer of water. Terracotta suits the plant because it dries between waterings, but a plastic pot with a gritty mix works just as well if you water by weight rather than by habit.
| Situation | Pot or spacing | Notes |
|---|---|---|
| Single rooted cutting, first potting | 3.5 inch (9 cm) pot | Gritty free draining mix; no feed for six weeks |
| Established single plant | 5 to 6 inch (13 to 15 cm) pot | Can stay two seasons before it needs moving up |
| Mixed container | 3 plants per 12 inch (30 cm) pot | Crowding is fine and improves flowering |
| Window box | 8 inches (20 cm) between plants | Restricted root run suits the plant well |
| Open ground bedding | 10 to 12 inches (25 to 30 cm) apart | Unimproved soil; do not manure the bed first |
| Trailing types in a basket | 3 to 5 plants per 14 inch (35 cm) basket | Plant round the rim so stems fall outward |
Compost and ground preparation
Use a compost that drains fast. A general purpose potting mix with a quarter to a third by volume of coarse grit or perlite mixed through it is exactly right, and it is the single easiest way to avoid the rot and yellow lower leaves that follow wet compost. A soil based compost with added grit is even better for plants staying two seasons, because it holds its structure.
Avoid anything sold as moisture retentive, and avoid water retaining gel crystals entirely. They do the opposite of what this plant wants. Do not add slow release fertilizer at planting either; the compost already holds six to eight weeks of nutrition and extra nitrogen delays flowering.
In the open ground, plant into soil as it is. Do not dig manure or rich compost into a bed for geraniums. If the ground is heavy and stays wet, fork grit through the top spade depth or plant on a slight ridge so water drains away from the crown. Geraniums tolerate poor, dry, chalky and stony ground better than almost any other bedding plant.
Do not plant the crown deeper than it sat in its pot. A geranium stem buried even half an inch (12 mm) too deep is a direct invitation to stem rot, because the buried section stays wet.
Planting and the first two weeks
Water the plant in its pot an hour beforehand so the rootball comes out intact. Make a hole just deep enough that the surface of the rootball sits flush with the surrounding soil, firm gently around it, and water in once, thoroughly.
Then hold back. The instinct after planting anything is to keep it damp for a fortnight, and with geraniums that is precisely wrong. Water once at planting and then not again until the soil has dried an inch (2.5 cm) down. Roots chase moisture downward when the surface dries, and a plant watered daily for its first two weeks builds a shallow root system it never recovers from.
If the plant is not already branched, pinch out the growing tip once it has settled and put out new growth, usually within ten days. That single pinch doubles the number of flowering stems. Do not pinch a plant that is already carrying buds.
- Check the roots before you plant Slide the plant out and look. White growing tips at the edge of the rootball means it is ready. A solid mat of coiled brown root means tease the bottom third loose before planting.
- Plant at the same depth it grew The junction of stem and compost must sit at the new surface. Burying it deeper is the commonest cause of stem rot in the first month.
- Water in once, then wait One thorough soak settles the soil around the roots. After that, water only when the ground has dried down an inch, even if the plant looks slightly limp at midday.
- Record what went where Note the planting date, the position of each plant, and which pot or bed it went into. When one group flowers better than another, the record explains why.